Although certain previously published documents are referred to in the specification, this is to assist the reader in understanding the specification; and none of these documents is admitted to be prior art against the present application. In particular, and without limitation, one or more of the documents referred to may be disclosures made by the inventor or joint inventor or by another who obtained the subject matter disclosed directly or indirectly from the inventor or a joint inventor and were published less than one year before the effective filing date.
Reference is now made to FIG. 1, which is a simplified schematic flow diagram for an exemplary water resource recovery facility, indicated generally by reference 100, which uses an activated sludge process. In such water resource recovery facilities, wastewater 102 is collected in lateral sewers serving both residential and industrial urban areas and these lateral sewers connect with larger trunk sewers that convey the wastewater 102 to the water resource recovery facility (also known as a wastewater treatment plant). As the wastewater 102 enters the water resource recovery facility it flows through bar racks and screens 110 to remove floating objects that can clog pumps and pipes, the objects that are removed are referred to as “screenings” 112. After the wastewater 102 has been screened it passes into a grit chamber 114 where sand, grit, and small stones 116 are removed from the wastewater 102. Screening and grit removal are known as preliminary treatment steps. With the screening and grit removal completed, the wastewater 102 still contains dissolved organic and inorganic matter and suspended solids. A portion of the suspended solids are removed using primary clarifiers 118 downstream of the grit chambers 114. In the primary clarifiers 118, the suspended solids have time to settle and are removed as raw sludge (also known as primary sludge) 120.
After preliminary and primary treatment, the wastewater 102 flows into aeration tanks 122 where the wastewater 102 is mixed with air and a concentrated stream of bacteria. The air is introduced using blowers that force air through pipes that feed diffusers within the aeration tanks. The wastewater 102 remains in the aeration tanks for several hours and the bacteria consume the organic matter contained in the wastewater. The organic matter is used for the synthesis of new bacterial cells and the production of the energy required for cell metabolism. A certain type of bacteria can also convert ammonia into nitrite and nitrate to provide its energy requirements in a process known as nitrification.
From the aeration tanks 122, the wastewater, which is now referred to as “mixed liquor” and denoted by reference 104, flows into secondary clarifiers 124 which remove the bacteria and other solids so that the treated effluent, denoted by reference 106, is suitable for downstream disinfection in a disinfection unit 126, such as a chlorination unit or a UV disinfection unit. The disinfected effluent 108 can then be discharged. In some cases, effluent may be discharged without disinfection.
The secondary clarifiers 124 also serve to thicken or concentrate the bacteria within the mixed liquor 104. This thickened stream of bacteria is known as the return activated sludge (RAS), denoted by reference 106A, and is recycled and mixed with the wastewater 102 leaving the primary clarifiers. Because the bacteria are continuously reproducing and the RAS stream 106A is recycling the bacteria, the mass of sludge will continue to increase without limit unless excess sludge, denoted by reference 166B, is wasted from the system 100. Excess sludge 166B is wasted or purged from the system by splitting off a portion of the return activated sludge 106A and sending it for thickening and stabilization and further volume reduction in aerobic or anaerobic digesters. The excess sludge 166B is known as waste activated sludge (WAS) 166B, and can alternatively be taken directly from the aeration tanks. In order to eliminate the need for thickener to concentrate the WAS, it can also be sent to the primary clarifiers.
The above description of an exemplary water resource recovery facility is provided to facilitate understanding of the context in which aspects of the presently disclosed technology may be applied, and is not intended to be limiting. One skilled in the art will appreciate that the water resource recovery facility described above is merely one exemplary implementation of such a facility, and that many variations and alternate arrangements are possible and that the presently disclosed technology may be applied to activated sludge processes in a wide variety of water resource recovery facilities.
Ammonia-based aeration control, also referred to by the acronym ABAC (Rieger et al., 2014), is a cascade control concept for controlling total ammonia nitrogen (NHx—N), which is the sum of NH3—N plus NH4—N in an activated sludge process. Its main goals are to tailor the aeration intensity to the NHx—N loading and to maintain consistent nitrification, which meets effluent limits but limits energy consumption and improves nutrient removal (Rieger et al., 2012).
One limitation to ABAC is that the solids retention time (SRT) control strategy used at a water resource recovery facility (WRRF) may not be consistent with the goals of ABAC. For example, ABAC may not be able to handle peak loads if the SRT is too low and may reach minimum airflow constraints if the solids retention time (SRT) is too high.